Castanyer sorts responses into three kinds — submissive, aggressive and assertive — describes where a lack of assertiveness comes from, and ends with a repertoire of techniques for specific situations. It is a popular book, with examples from personal life, and it has run to more than thirty printings for one reason: the problem it addresses is universal. It has not been widely translated, which is a pity, because the English-language shelf on this subject is mostly worse.
What I took from it
First, an uncomfortable diagnosis: people who work for themselves lean submissive with clients, not out of shyness but out of fear of losing the work. A request outside the agreed scope gets accepted, the cost of it never gets said, and a month later the project is late and the resentment surfaces somewhere else. The book's description of how accumulated submission ends in an aggressive outburst maps that sequence precisely.
Second, the operational part. An assertive reply is not a refusal: it is a sentence saying what is included, what is not, and what the missing part would cost. No long justifications, which are the polite way of asking permission. Working that way, when a request drifts outside the agreement I answer with that three-part shape and nothing else, and the conversation gets shorter and easier for both sides.
Third, the chapter on non-verbal components. You can deliver a perfectly assertive sentence in an apologetic tone, and what lands is the apology. On a call, the tone in which a price is said communicates more than the price.
What I left out
The self-esteem framing, which is the most dated part of the book and the one that reads most like nineties self-help. The exercises are built for a therapy group and for personal relationships, not for a scope negotiation. You get more from the third of the book that is technique than from the two thirds that are theory.